CHANGELOG: Collapse README.md headings in single list

While it's not happening today, we'll want to eventually also handle
updates to this CHANGELOG list via krel changelog. By collapsing the set
of changelogs into a single list, it will be a little easier to write
out an updated file for commit by searching the contents of the directory
that match "CHANGELOG-x.y.md" and adding them to the list.

Signed-off-by: Stephen Augustus <[email protected]>

Author: justaugustus

CHANGELOG/README.md

@@ -1,15 +1,7 @@
 # CHANGELOGs
 
-## Development release
-
 - [CHANGELOG-1.18.md](./CHANGELOG-1.18.md)
-
-## Current release
-
 - [CHANGELOG-1.17.md](./CHANGELOG-1.17.md)
-
-## Older releases
-
 - [CHANGELOG-1.16.md](./CHANGELOG-1.16.md)
 - [CHANGELOG-1.15.md](./CHANGELOG-1.15.md)
 - [CHANGELOG-1.14.md](./CHANGELOG-1.14.md)